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Driveability Corner

Intermittent driveability problems are among the toughest to diagnose and repair. This one is no exception, and it will take more than one column before we've finished unraveling it.

I'm working on a 2000 Dodge Ram 2500 Van with the 5.9L engine. The complaint is misfire, intermittent stall, no restart and a NO BUS light on in place of the odometer reading. The diagnostic tree starts with checking for no communication with a scan tool, powertrain control module (PCM) power inputs, then checking the 5-volt sensor power supply and finally disconnecting and connecting sensors and various loads while checking if communication is restored. The diagnostic ends with using an ohmmeter to check the wiring for opens and shorts.

The list of possible faults given for the symptoms is fairly long:

•fused ignition switch output circuit open

•PCM ground circuit(s) open

•NO RESPONSE display from DRB scan tool

•ASD relay output circuit shorted to ground

•camshaft position sensor

•crankshaft position sensor

•fuel injector(s)

•generator shorted to ground

•ignition coil

•leak detection pump

•MAP sensor

•O 2 sensor(s)

•TPS (5-volt supply)

•TPS ground circuit shorted to voltage

•transmission solenoid

•PCM 5-volt primary supply circuit shorted to ground

•PCM 5-volt secondary supply circuit shorted to ground

•fused B+ circuit open between fuse and battery

•fused B+ circuit open between fuse and PCM

•fused B+ circuit shorted to ground

•powertrain control module (5-volt)

Here's the rub: The problem is intermittent. Usually, I can get the symptoms to occur after 20 minutes of run time. Sometimes it takes two hours to duplicate the problem. The problem may last 20 seconds to 5 minutes, not enough time to successfully complete the factory-specified diagnostics. Okay, I really didn't like the time it looked like the diagnostic would take, even with a consistent fault. Time to break out the scope and look at some waveforms.

I've got an eight-trace scope, so I figured I'd monitor the following signals (most at the PCM):

•battery positive (B+)

•switched ignition power

•5-volt sensor supply

•one injector circuit

•CC­–

•CCD+

•current probe monitoring all switched power through Auto-Shut-Down (ASD) relay.

First I recorded “good” waveforms when the van was running fine. Everything looked pretty normal, except the 5-volt reference was varying wildly and went as high as 5.3 volts. Check out Fig. 1 on the opposite page. The PCM's 5-volt reference regulator should be able to control the voltage within .010 volt, or 10 millivolts (mV). So why the voltage variation? Notice that despite the wide variation, the voltage never drops below 5 volts. The PCM has a poor ground reference, which we know causes a rise in voltage. The variation is relative to the ground resistance and the current flow through the ground. Rapidly switched high-current circuits, like injectors and ignition, cause the voltage variation across the ground.

Next I decided to do a “deep recording.” Look at the time scale at the bottom of Fig. 2 (600 seconds, or 10 minutes). The PC-based scope I used recorded 250,000 samples in the 600 seconds, or 416 samples per second. With some signals measured in milliseconds (1000 per second), you can see the sample rate is not great for zooming in and looking at signals in great detail. The sample time is great for catching the stall event and looking at what happened to my signals before and after the event.

In Fig. 2, look at the engine running event from 290 to 325 seconds (the point of stall). What do we know? First, the battery supply voltage and switched supply voltage are normal. Both voltages (red and yellow traces, sometimes on top of each other) are normal charge voltages while running and slope down to battery voltage at the point of stall. Also, if you're perceptive you'll see that the injector supply voltage is also the same as B+ and switched voltage. We know the ignition switch and other connections have very little voltage drop relative to these circuits.

In Fig. 2 we can see some variation in the 5-volt reference voltage. Fig. 1 is a zoom in from the same recording of the 5-volt reference.

Let's move to Fig. 3, a zoom in of the stall point in Fig. 2. What can we determine from the injector waveform? The engine misfires badly before the stall, often for quite a long time. Note the missing injector events. At this sample rate you've got to be careful; the scope may miss some events. But you can see that, as the engine misfires, injector events are legitimately missing. Hmm. When the stall occurs, what happens to the injector signal voltage? It drops to zero. Hmm. Remember, the injector circuit is powered by 12 volts and pulled to ground by the injector driver in the PCM. Why did the injector circuit voltage drop to zero with the key on? The Auto-Shut-Down relay powers the injectors, ignition, oxygen sensor heaters, etc. The ASD is controlled by the PCM based on rpm input. What could cause the intermittent loss of injector signal and the shutdown of the ASD? Looks like there could be a problem with the crank sensor signal to the PCM. Is the ground problem causing the issue with the crank sensor signal?

In Fig. 3, note that the 5-volt reference (blue trace) is above 5 volts. Now let's look at the CCD– (white trace) and CCD+ (purple trace). According to MOTOR/ALLDATA, the CCD bus voltages should lie between 1.8 and 2.8 volts. Looks like the CCD– is all over the place and outside acceptable voltage levels. Is the CCD– affected by the ground problem also? It's interesting to note that the NO BUS indicator never came on until after the stall and when the no-start occurred. The CCD– is whacked out, often with no driveability problem, no stall and no NO BUS indication.

Next time we'll look at the crank sensor waveform and the ASD supplied circuit's current waveform, conduct the factory ohm test for the grounds and see what happens with clean grounds. Stay tuned.